This invention relates to apparatus for use in an aircraft, and is particularly adapted for and intended to be used in small aircraft. The purpose of the apparatus of the present invention is to provide a device which will, at any instant in time, determine the ambient temperature and barometric pressure of the air where the aircraft is presently located, whether it is flying or on the ground, and to provide a readout of pressure densityxe2x80x94otherwise also known as density altitude. The present invention may also provide further displays for specific aircraft performance characteristics, at any instant in time.
Any aircraft pilot has, of course, great concern about the performance of the aircraft that he/she is flying. Accordingly, the pilot relies on a great variety of instruments which are mounted in the cockpit of the aircraft within his/her field of view. Of course, in larger multi-engine aircraft, and especially commercial aircraft, as well as in high performance aircraft such as military aircraft, there may be very extensive instrumentation. On the other hand, in small single, two-seater or four-seater, single-engine aircraft of the sort used by recreational flyers, bush pilots, and the like, there may be a paucity of instrumentationxe2x80x94the aircraft being provided with sufficient instrumentation to permit it to be safely flown, as determined by the manufacturer of the aircraft.
However, no aircraft is provided with an instrument which will give a dynamic reading of pressure density, which is sometimes referred to as density altitude. Knowledge of the pressure density at any instant in time is required to determine requirements or flying characteristics such as ground roll necessary for safe takeoff of the aircraft, or for a determination of true air speed over ground. Other performance characteristics are more particularly described and discussed hereafter. However, it must be understood that it is the importance of density altitude, and the manner in which it affects other readings, which is primarily being dealt with.
Of course, it is well understood that the higher the altitudexe2x80x94generally, altitude is determined as being the altitude above sea levelxe2x80x94the less dense the air. Likewise, the warmer the air becomes, the less dense it will become. It must also be understood that what is called an altimeter in an aircraft, especially a small aircraft, is not in fact an instrument which measures precise altitude above sea level. In fact, the altimeter is actually an aneroid barometer which measures atmospheric pressure. There is, therefore, an indicated altitude, but that indicated altitude must be corrected for local conditionsxe2x80x94a process which is well known to aircraft pilots, particularly as they are preparing for takeoff. Especially, the flight altimeter settings for the aircraft must be adjusted by the pilot to the airport elevation and station pressure, with the current ambient temperature having to be taken into account when the pilot is calculating the length of the ground roll which is required for takeoff.
Moreover, as will be discussed in greater detail hereafter, atmospheric pressure and temperature conditions are dynamic, and are constantly changing. It is well known that atmospheric pressure and temperature will affect flight performance of the aircraft, as well as its takeoff and landing conditions. Thus, the need for dynamic and real time knowledge of the pressure density becomes understood.
There are a number of different readings or indicators of altitude which may be referred to or required to be known at any instant in time by the pilot of an aircraft. Again, it must be understood that an altimeter in an aircraft is calibrated to show height above sea level under standard atmospheric conditions. Standard atmospheric conditions are 29.92 inches of mercury and 59xc2x0 F. However, local conditions of temperature and pressure will most likely not match the standard conditions.
Indicated altitude is the altitude which is shown on the altimeter of the aircraft. If the altimeter is set to the current atmospheric pressure, corrected to sea level, the indicated altitude will be approximately equal to the height of the aircraft above sea level.
Pressure altitude is the altitude which is shown on the altimeter when the pressure is set to 29.92 inches of mercury.
Density altitudexe2x80x94or pressure density, as it referred to hereinxe2x80x94is the pressure altitude which is corrected for deviations from standard temperature. It is important for the pilot to know the pressure density or density altitude in order for him/her to calculate the required runway for ground roll in order to takeoff, and to determine the rate of climb of the aircraft once it has taken off. Particular embodiments of the present invention will provide those data automatically to the pilot, upon an appropriate query and input of necessary parameter data to the apparatus of the present invention.
It will be understood that takeoff on a hot day from an airport with an elevation well above sea level will require much greater ground roll than a takeoff from an airport at sea level on a cold day.
True altitude is the actual height of the aircraft above sea level. If the altimeter in a small aircraft has been set to local pressure, corrected to sea level, than the indicated altitude is approximately the true altitude of the aircraft above sea level.
The other two types of altitude, absolute altitude and radio or radar altitude, require that the aircraft be equipped with a radio or radar altimeter, and are beyond the scope of the present discussion.
The effect of normal pressure variations on true altitude may be quite profound. Pilots are warned to always recall that pressure variations will change from time to time, as they fly across country, as the day warms up or cools down, or as a weather front may be moving into the region where the aircraft is operating. If a pilot is flying the aircraft having a constant indicated altitude, the aircraft is, in fact, being flown in a constant barometric pressurexe2x80x94the aircraft is following an isobaric profile. Thus, if the aircraft is flown at a constant indicated altitude into an area of lower barometric pressure, it is flown xe2x80x9cdownhillxe2x80x9d into a pressure valley; and, if the aircraft is flown into an area of higher of barometric pressure, it climbs a pressure hill.
Pilots of small aircraft that fly into an area of low pressure may notice a pressure drop of as much as 0.5 inches of mercury over a distance of as little as 200 miles in a severe weather front. Since atmospheric pressure above a given land point will decrease by about 0.1 inch of mercury per 100 feet of altitude, the pressure effect can be quite profoundxe2x80x94in the example given above, as much as 500 feet. Moreover, as the temperature changes, the density of the air will also change; therefore, flying into a low pressure area on a warm day, with the temperature rising, may indeed have profound affects on the flying characteristics of the aircraft and particularly on a determination of where the aircraft is actually located in altitude.
Examples of the manner in which temperature will affect pressure density or density altitude are now given. As stated, the international standard for zero feel of pressure density or density altitude is 59xc2x0 F. at sea level and 29.92 inches of mercury. However, at sea level and 29.92 inches of mercury, if the temperature rises to 80xc2x0 F., the pressure density will rise to 1,200 feet. In other words, the same air density will occur at sea level and 29.92 inches of mercury at 80xc2x0 F. as will occur had the aircraft taken off from sea level at 59xc2x0 F. and 29.92 inches of mercury and climbed to 1,200 feet. Likewise, as temperature goes down, pressure density will go down. For example, if the temperature is 52xc2x0 F. and the barometric pressure is 29.92 inches of mercury at 2,000 feet, the pressure density will also be zero feetxe2x80x94that is, the same conditions prevail as they did at sea level and 59xc2x0 F. and 29.92 inches of mercury.
Another example is that, at 8,000 feet true altitude above sea level, and 80xc2x0 F., the pressure density will be 11,100 feet; whereas, as sea level, the pressure density will only be 1,200 feet.
Obviously, therefore, an increase in pressure density or density altitude will require an increased takeoff distance, and result in a reduced rate of climb once the aircraft has taken off. Moreover, the increase in pressure density will result in an increased true air speed on approach and landingxe2x80x94lift reduces as air density reducesxe2x80x94and it will require a longer landing roll distance as well.
The necessity for a device which will give dynamic readings of pressure density, or density altitude, becomes clear. Such a device that will operate in real time so that, at any instant in time, the pilot may determine the existing condition of pressure density, allows for much safer operating conditions of the aircraft, and such an instrument is provided by the present invention.
Thus, when the pilot of an aircraft is able to determine dynamic pressure density readings at any time throughout the flight of the aircraft, the pilot will have an improved overview of the current operating limitations of the aircraft. Thus, the pilot will have a much greater understanding of the true air speed of the aircraft, the ground roll required for takeoff or landing, the service ceiling beyond which the aircraft should not fly, and so on.
Briefly, a pressure density determining apparatus must be able to sense the current ambient temperature and the current ambient barometric pressure, and the device must be able to correlate the two so as to calculate the value of pressure density. The correlation is in the form of a calculated lookup table for a broad range of pressure values, as discussed hereafter. Briefly, however, this will permit the pilot to query the apparatus of the present invention, after the apparatus has sensed the current operating ambient temperature and barometric pressure values so that, expected ground roll at that given pressure density for that aircraft may be determined. If so, then the pilot is aware of the minimum required runway length for takeoff.
As discussed hereafter, the device of the present invention may also be used to calculate true air speed at a given power setting. Once a pressure density value has been calculated, the indicated air speed may be input into the device, and a waiting factor assigned to it, which is determined as a function of the pressure density, so as to determine true air speed.
CLEM et al. U.S. Pat. No. 3,839,626 teach an altimeter setting indicator which is intended to specifically provide a means for converting measured airport barometric pressure to altimeter setting barometric pressure. This device is used to provide remote digital readouts for reporting by the airport tower and air traffic control centres, but the device is not used on the aircraft per se. The device uses a barometric pressure sensor to provide a signal to a convertor, which will convert that signal into a corresponding day pressure altitude signal. A computer which is responsive to the local pressure signal as well as a signal proportional to the local elevation will then provide signal which corresponds to the altitude difference between the two. A second convertor then responds to this altitude difference and converts it into a signal which is proportional to the corresponding day barometric pressure and provides the required altimeter settings. The data acquired by the barometric pressure sensor is manipulated by a multitude of converters so as to provide an accurate measure of the required altimeter setting pressure to be used by air traffic control towers and/or air traffic control centres who will then relay that information to the pilot of an aircraft for use by the pilot to set a corrected altimeter reading on the instrument panel of the aircraft.
YOUNKIN U.S. Pat. No. 4,008,618 teaches a flight instrument which has both analog and digital display means using a rotary drum digital indicator. The aircraft instrument has a barometric pressure responsive transducer to generate barometric pressure change signals. The dynamic barometric pressure is converted into rectilinear motion, which is used to drive a mechanical-to-electrical transducer which provides a signal to a servo-amplifier so as to drive the rotary drum digital indicator. Means are provided to correct for atmospheric along a given flight path, with respect to sea level.
U.S. Pat. No. 4,133,503 issued to BLISS teaches a method and apparatus by which a pilot can control the speed of an aircraft on the landing approach, where the speed designated eliminates, as much as possible, a wind shear hazard. A conventional central air data computer is used to compute the true air speed by applying temperature and pressure corrections to the indicated airspeed. A safe, stable speed may then be used during landing approaches, and the pilot will be kept apprised of the conditions through which the aircraft is flying during its landing approach.
SEEMANN U.S. Pat. No. 4,263,804 teaches an apparatus which is intended to directly measure the density altitude of an aircraft. However, here a device is taught which requires an electric motor connected to drive a rotor by which an air current is drawn into the apparatus and is constantly sampled, amplified, and changed in shape, and applied as an input signal to a display device which is calibrated to display density altitudexe2x80x94i.e.: pressure density. In order to calculate the density altitude, an amplifier/shaper monitors changes in operational characteristics of the motor. This produces a continuous monitoring signal, which is applied as an input signal to a display device appropriately calibrated.
U.S. Pat. No. 4,980,833 issued to MILLIGAN et al. teaches a takeoff monitor having a learning featurexe2x80x94a smart monitor that learns the takeoff characteristics of the aircraft over a period of time. The monitor maintains historical data of the takeoff performance of the aircraft. The monitor analyses the performance of the aircraft during its takeoff through a movement signal and using the movement signal concomitantly with the historical data allows the pilots to judge the adequacy of takeoff performance during takeoff. The monitor is equipped with a display, an input panel, function keys, and numerical keys; and the input data which are required for the calculation of an acceleration curve including both temperature and pressure. However, the instrument is incapable of sensing any of the required input parameters, and merely provides a database for past performance whereby current performance can be evaluated.
ZIMMERMAN et al. U.S. Pat. No. 5,001,638 teaches the use of a plurality of first and second sensors so as to determine the flight status of an aircraft. These sensors include static pressure sensors, total temperature sensors, and total pressure sensors. However, the patent is particularly directed to monitoring of an engine control system in an aircraft.
MIDDLETON et al. U.S. Pat. No. 5,047,942 teaches a real time takeoff and landing performance monitoring system which is intended to be responsive to various ambient conditions. Temperature and pressure values may be input from transducers, or manually, using a navigation control display unit. This system continuously monitors takeoff and landing performance by comparing the actual performance of the aircraft with nominal performance. The system will generate values for required takeoff roll, instantaneous speed, and acceleration. The apparatus employs the use of lookup tables; and a head-up or head-down display device is employed.
MORBIEU U.S. Pat. No. 5,648,604 describes a method for determining anemobaroclinometric parameters onboard an aircraft. Static pressure and temperature values are obtained using sensors; and, using the temperature and pressure data, density and altitude pressure may be determined through mathematical modes, and estimates of airspeed vector may also be calculated. Two principal computers are required as a means of data delivery.
In accordance with one aspect of the present invention, there is provided an apparatus which, in general, is physically quite small and which may be hand-held. The apparatus of the present invention is essentially portable, although it may be configured for installation into one specific aircraft. Otherwise, the present invention provides an apparatus which, because it is portable, may be used by such as xe2x80x9cweekendxe2x80x9d pilots who may fly recreationally, and who are required to rent an aircraft from their flying club.
A purpose of the present invention is to provide an apparatus whereby the pilot, whether he/she is a student pilot, a general aviation pilot, or perhaps a professional pilot such as a bush pilot, may enjoy a greater understanding of atmospheric conditions in which the aircraft is operating, and therefore will have a greater understanding of what aircraft performance is to be expected for that aircraft under the present operating conditions.
The present invention provides an apparatus for use in an aircraft for determining and displaying a reading on a display, where the reading is indicative of pressure density at any instant in time. The apparatus comprises sensing means for determining ambient air pressure and outputting an electrical signal having a value which varies in accordance with the ambient air pressure at any instant in time, and sensing means for determining ambient air temperature and for outputting an electrical signal having a value which various in accordance with the ambient air temperature at any instant in time. Calculating means are provided for combining the electrical signals from the sensing means for ambient air pressure and ambient temperature, and outputting a weighted signal having a value which is based on an index value of zero for a pressure density at sea level when the temperature is 59xc2x0 F. and the ambient pressure is 29.92 inches of mercury.
Microprocessor computer means are provided having both random access memory and read only memory, and having an output signal driver means for outputting a signal which is indicative of pressure density at any instant in time. A display means is provided for displaying a value of pressure density at any instant in time, based on the value of the signal received from the output signal driver means.
The microprocessor computer means includes a lookup and comparator means. The read only memory contains a plurality of calculated values of pressure density over a range of barometric pressure values chosen from the limits from 28.5 inches of mercury to 31.5 inches of mercury, and a range of temperature values chosen from the limits of from xe2x88x9240xc2x0 F. to +130xc2x0 F.
The weighted signal from the calculating means may be read by the comparator and lookup means, and compared with values of pressure density which are stored in the read only memory, so as to determine the value of pressure density at that instant in time, as represented by the weighted signal.
The determined value of pressure density at that instant in time is fed to the output signal driver means so as to cause the display means to display the determined value of pressure density at that instant in time.
Generally, the calculating means is included in the microprocessor computer means. In other words, the calculating means is generally a mathematical operator programmed into the microprocessor so as to take digital electrical signals having values which are indicative of ambient air pressure and ambient air temperature at any instant in time, and calculating a weighted signal which is indicative of pressure density, where a signal having a value of zero is representative of pressure density at sea level when the ambient temperature is 59xc2x0 F. and the ambient pressure is 29.92 inches of mercury.
On the other hand, the calculating means may be such as a Wheatstone bridge, which is set up with appropriate resistance values so as to provide an electrical signal from its output terminal which output signal is determined from analog signals indicative of ambient air pressure and ambient air temperature at any instant in time. Of course, in such circumstances, an analog to digital convertor will be required to input a meaningful signal indicative of the instantaneous value of pressure density, for further handling by the microprocessor computer means.
So as to make the apparatus of the present invention useful beyond simply determining an instantaneous reading, in real time, of pressure density, the apparatus of the present invention is provided with input means for inputting a selected parameter at any instant in time to the microprocessor computer means. The selected parameter may be one which is chosen from the group consisting of present standing :altitude, aircraft weight, and indicated airspeed.
The input means may comprise a numeric keyboard which will, itself, include appropriate scrolling or function keys, whereby numeric values of present standing altitude, aircraft weight, and indicated air speed may be entered into the microprocessor computer.
On the other hand, the input means may comprise a selector key and a scroll key. In that case, the read only memory of the microprocessor computer will include at least one parameter value table chosen from the group of parameters consisting of present standing altitude, aircraft weight, and indicated air speed. Thus, the specific relevant values of present standing altitude, aircraft weight, or indicated air speed may be chosen from a lookup table using the selector key and scroll key, as necessary.
The read only memory may also include at least one aircraft performance characteristic lookup table generally chosen from the group of aircraft performance lookup tables consisting of takeoff distance, landing distance, rate of climb, time to climb, fuel to climb, and distance to climb.
Thus, at any instant in time, the present value of pressure density may be determined and fed to the microprocessor, and a selected parameter may be input to the microprocessor. Then, the values of pressure density and the selected parameter may be read by lookup means in the microprocessor so as to determine a specific correlated value from a chosen one of the aircraft performance characteristic lookup tables. That specific correlated value of the chosen one of the aircraft performance characteristics may then be fed to the output signal driver so as to output a signal indicative thereof to the display means. Accordingly, the display means will show a reading of the chosen aircraft performance characteristic for the pressure density conditions at that instant in time.
The aircraft performance characteristic lookup tables are generally specific to a particular aircraft type. However, depending on the size of the read only memory which may be installed in the microprocessor computer, it may be that there are aircraft performance characteristic lookup tables included in the read only memory which are specific to a chosen group of particular aircraft types. As an example, tables for aircraft performance characteristics of takeoff distance, landing distance, rate of climb, time to climb, fuel to climb, and distance to climb, may be provided for a group of aircraft manufactured by Cessna, or by Beechcraft, so that a flying club or small operating aviation company which owns and/or operates a number of different aircraft of the same general type, manufactured by the same manufacturer, or more especially a pilot which may fly any one of those different aircraft, may employ the use of a single pressure density determination apparatus in keeping with the present invention, for use with differing aircraft.
Typically, although the following discussion is by way of example only, calculated values of pressure density over a range of barometric pressure values and a range of temperature values will be provided over ranges which may be as broad as from 28.5 inches of mercury to 31.5 inches of mercury for barometric pressure, and from xe2x88x9240xc2x0 F. to +130xc2x0 F. for air temperature. Moreover, the steps within each of the ranges may vary, but typically steps of 0.2 inches of mercury and xc2x15xc2x0 F. will be sufficient for purposes of providing a meaningful reading of pressure density to the pilot. Indeed, steps as high as 0.3 or 0.4 inches of mercury, and xc2x110xc2x0 F. may be acceptable.